1. Technical Field
The present disclosure relates to improvement in the light extraction efficiency of ultraviolet light-emitting diodes.
2. Description of the Related Art
Solid state light-emitting elements utilizing a nitride semiconductor material have been widely used in applications of blue light-emitting diodes (LEDs). Similar solid state light-emitting elements are in demand for use in shorter wavelength applications. Thus, ultraviolet light-emitting diodes (UVLEDs) have been developed based on material groups similar to those of blue LEDs. Since ultraviolet (UV) light is considered to have a variety of useful applications including sterilization, water purification, and medical applications, especially so for light at short wavelengths in the deep ultraviolet range of 350 nm or below, or more specifically, in a part of the UVC range of around 260-280 nm, LEDs that operate in the UVC range, and deep UV LEDs (DUVLEDs) are under intensive development. A typical DUVLED uses a sapphire substrate or a single crystal AlN substrate and has a layered structure made of a gallium aluminum nitride series semiconductor containing aluminum (Al), gallium (Ga), and nitrogen (N) for its main composition. Output power of such DUVLEDs has been improved, and DUVLEDs that output at 10 mW output level UV radiation have been manufactured to date.
Technological challenges for such DUVLEDs include improvement of emission efficiency, as an example. The emission efficiency may be measured by an external quantum efficiency, ηEQE, which is defined as the number of photons emitted by the LED element per unit time, divided by the number of electrons in the operational electric current per unit time. The external quantum efficiency, ηEQE can be factored into three factors of an internal quantum efficiency ηIQE, an electron injection efficiency ηEIE, and a light extraction efficiency ηLEE, given by the following relationship:ηEQE=ηIQE×ηEIE×ηLEE.
Among the three factors for the DUVLEDs the internal quantum efficiency ηIQE and the electron injection efficiency ηEIE have been drastically improved to date as a result of continuing development efforts. Specifically, technological solutions that have contributed to such improvement include reduction of crystalline dislocations for the ultraviolet emission layer to improve the internal quantum efficiency ηIQE (see, for example, U.S. Pat. No. 7,811,847; H. Hirayama et al., “231-261 nm AlGaN deep-ultraviolet light-emitting diodes fabricated on AlN multilayer buffers grown by ammonia pulse-flow method on sapphire,” Appl. Phys. Lett. 91, 071901 (2007); H. Hirayama et al., “222-282 nm AlGaN and InAlGaN-based deep-UV LEDs fabricated on high-quality AlN on sapphire,” Phys. Stat. Solidi (a), 206, 1176, (2009); and S, Fujikawa et al., “Realization of 340-nm-band high-output-power (7 mW) InAlGaN quantum well ultraviolet light-emitting diode with p-type InAlGaN”, Jap. J. Appl. Phys. 47, 2941 (2008)). For the improvement of the electron injection efficiency ηEIE, it is effective to supplement electron blocking performance in a p-type semiconductor layer by adopting a structure called an MQB (multi-quantum barrier) that utilizes a superlattice structure in the p-type semiconductor layer (see, for example, U.S. Pat. No. 8,759,813). Currently as much as 50-80% efficiency can be expected for a product of the internal quantum efficiency and the electronic injection efficiency, or ηIQE×ηEIE.
However, even for DUVLED elements for which a high value of the product of the internal quantum efficiency and the electron injection efficiency ηIQE×ηEIE, fabricated DUVLED elements have been operated at an external quantum efficiency ηEQE of around 4% at the maximum. Such poor performance can be attributed to an insufficient value of the light extraction efficiency, ηLEE. Light extraction efficiencies ηLEE for conventional DUVLED are typically less than 10%.